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I have a Lenovo Thinkpad x1 carbon with a WQHD display (2560x1440) running Linux Mint 18.2 Cinnamon with the Double HiDPi UI scaling enabled, as well as an xrandr configuration startup script that sets the scaling more to my liking. When waking from suspend (lid closed) the scaling remains but the virtual window shrinks, causing the display to be only partially filled (i.e. about 2/3 of it is black with a smaller version of my desktop in the upper left region).

I implemented a systemd service that calls xrandr --auto upon resuming from suspend. It fixes the problem after it occurs but seems like a rather ham-fisted solution. I would like to know why this issue happens in the first place, especially considering that it only happens after resuming from suspend with the lid closed, not from the menu or command line. Is it a bug? An issue with the laptop's compatibility? Something going on with X Server? I've tried a number of troubleshooting steps, and I haven't made any other scripts besides the aforementioned that modify the display properties, so it has to be something with how Mint handles laptop-related events. Any ideas?

Initial startup:

xrandr --output eDP1 --scale 1.25x1.25 --fb 3200x1800 --panning 3200x1800

Resume command (called in a systemd unit)

xrandr --auto

Per the Linux Mint 18.2 release notes:

Known Issues Cinnamon freezes when changing resolution (Intel GPU)

If your Intel GPU is recent enough (2007 or newer) it is recommended
to remove the legacy Intel driver "xserver-xorg-video-intel" and to
use the built-in modesetting driver instead:

apt remove xserver-xorg-video-intel
sudo reboot

Not only will the modesetting fix the freeze issue when changing
resolutions, it should also work better with Cinnamon and produce
better performance.

The legacy driver is no longer maintained and only useful for the old
i800x and i900x family of chipsets.

This did the trick, for anyone else who has this excellent and (mostly) Linux-ready laptop.